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I'm very sorry to hear about your situation. First of all, South Carolina law is going to apply because that's where you're based, not where the employer is based (otherwise every employer in the nation would be based in the most employer-friendly state).
Next you need to understand that South Carolina is an "at will" employment state. At-will employment means that without a contract, you have no contractual or other right to employment with the company. The company is entitled to fire you for any reason: a good reason, a poor reason, or no reason at all--as long as the company does not fire you for an illegal reason (race, gender, age, religion, etc...). But it extends beyond firing, to hiring, promotions, demotions, wage cuts and raises, disciplinary actions, and even scheduling. Unless you can show that this was done in violation of a contract, union agreement, or a clear violation of an unambiguous and binding clause against the employer, or that it was done because of some minority status (age, race, gender, religion, disability) that you have, then they do have this discretion. They can based the wages on a 45 hour work week, so long as they pay you for the time that you actually work. If you're working hourly, they have to pay you the hourly wage up to 40 hours, and 1.5x that hourly wage for every hour after 40 hours a week. This is something they can legally do because of the at-will doctrine. (By the way, Rhode Island is also an at-will employment state, so it wouldn't change anything if Rhode Island law were applied).
In your case, you are covered by Title VII and the ADEA (Age Discrimination in Employment Act), meaning that you could, potentially, have a case and complaint against your employer. That being said, age discrimination cases are difficult to win. In order to show a prima facie case for age discrimination in reductions in force, you are going to have to show: (i) your were in a protected age class (check), (ii) you were discharged or suffered an adverse employment decision (check), (iii) at the time of your discharge / adverse employment decision, you were performing your job at a level that met your employer's legitimate expectations (likely), and (iv) persons of your qualifications were retained in the same position or there was some other evidence that the employer did not treat age neutrally in deciding to dismiss plaintiff or have that adverse employment action (more difficult).
In ADEA ("age discrimination") cases, the plaintiff must present evidence demonstrating a "reasonable probability" and not merely a "possibility" of age discrimination. First, a plaintiff can offer direct and circumstantial evidence that he would not have been discharged but for his age; the evidence must be of sufficient probative force to support an inference of discrimination. Second, a plaintiff may use the McDonnell Douglas scheme of shifting burdens applied in Title VII cases. Under this scheme, once the plaintiff has established a prima facie case of age discrimination, the burden shifts to the employer to articulate a legitimate nondiscriminatory reason for its employment decision. If the employer meets this burden, the plaintiff must then show that the employer's proffered reason was mere pretext and that age was the more likely reason for his dismissal.
The Supreme Court has expressly held that an employer does not violate the ADEA "by acting on the basis of a factor, such as an employee's pension status or seniority [or salary], that is empirically correlated with age." Even though age is often related to factors such as salary, it is "analytically distinct" from them. For this reason, "an employer can take account of one while ignoring the other." Moreover, "[w]hen the employer's decision is wholly motivated by factors other than age, ... [e]ven if the motivating factor is correlated with age," that decision is not contrary to the ADEA. Accordingly, even if the company reduced the salary to reduce its costs (or even fired The employee simply to reduce its salary costs) this is not evidence of age discrimination.
IF others have been affected the same as you, and there's no strong indication of age discrimination, an age discrimination case probably wouldn't work. You'd have to show that you were singled out, or at least a class of you were singled out, and that the strongest, if not sole, reason for the discrimination was due to age. If you can't show that, I'm sorry to say that their actions would be legal. Unethical, immoral, and illogical maybe, but legal.
I know this is probably not what you wanted to hear, but it is the law. I hope that clears things up anyway. If you have any other questions, please let me know. If not, and you have not yet, please rate my answer AND press the "submit" button, if applicable. Please note that I don't get any credit for the time and effort that I spent on this answer unless and until you rate it positively (3 or more stars). Look for the stars on your screen (★★★★★). Thank you, ***** ***** luck to you!